The present invention relates to a powdery graft-copolymer blend possessing improved powder characteristics. More particularly, the invention relates to a powdery graft-copolymer with improved powder characteristics obtained by blending in slurry form or dry state a given ratio of graft-copolymers (A) and (B) comprising different quantities of elastomeric trunk polymers, each of the graft-copolymers having been obtained by emulsion graft-polymerizing a hard-resin producing monomer on the elastomeric trunk polymer.
Hard resins such as vinyl chloride resins, styrene resins, acrylonitrile-styrene resins, and methyl methacrylate resins are widely used because of their rigidity, transparency and processability. However, these resins are very brittle on the other hand, and thus are generally blended with graft-copolymers obtained by adding acrylonitrile and styrene, or methyl methacrylate and styrene to an elastomeric trunk polymer such as polybutadiene and subjecting both the monomers and the trunk polymer to polymerization (hereinafter referred to as ABS resins or MBS resins) in order to obtain improved impact strength.
The graft-copolymers of this type are widely used because of their remarkable effect of improving impact strength without impairing the desired properties of the resin to be blended therewith. These graft-copolymers are prepared through emulsion polymerization, and the latex obtained is coagulated with an acid or a salt, and then dehydrated and dried to obtain a powder. The graft-copolymer in powder form often gives rise to difficulties in handling due to agglomeration of powders during storage, i.e. a blocking phenomenon, or clogging of transportation lines caused by poor fluidity. In recent years when time and labor saving is encouraged by the powder weighing automation system and mass transportation systems, an improvement in powder characteristics including a blocking property, fluidity and bulk density is urgently needed.
To date, various methods for improving powder characteristics have been proposed. Examples of such methods are: those which involve granulating by spray-drying a graft-copolymer latex;
those which involve controlling conditions under which a copolymer latex is coagulated with an acid or a salt (coagulating conditions); or those which involve treating the surface of a copolymer powder with various additives such as lubricants and plasticizers.
However, these prior art methods have serious drawbacks in that the modified graft-copolymer obtained still possesses unsatisfactory powder characteristics or that, even if its powder characteristics are improved, its innate properties deteriorate inasmuch as its impact strength or effect of imparting high impact strength to a hard resin is lowered, or its transparency, processability or heat stability is impaired.